1. Field of the Invention
The present invention relates to a process for the extrusion of a thermoplastic polyimide and a polyimide pellet used for the process. More particularly, in the extrusion of polyimide, the invention relates to a process which can continuously produce articles without bubbles or die tailings at a constant extrusion rate, that is, an extrusion process for previously crystallizing a polyimide of a specific structure into a specific range of crystallinity and feeding the polyimide to an extruder, and relates to the polyimide pellet used for the process.
2. Prior Art of the Invention
Aromatic polyimides exhibit outstanding heat resistance in comparison with other organic polymers and additionally have excellent mechanical properties, chemical resistance and electrical insulating properties. Hence, aromatic polyimides are widely used in such fields as an electric and electronic industry, mechanical industry, atomic industry and automotive industry.
Thermoplastic polyimides and processing methods thereof have recently been reported.
In these thermoplastic polyimides, polyimides consisting of recurring units represented by the formula (I): ##STR2## wherein X is a single bond or a hexafluoroisopropylidene group, have been developed to have good extrudability as disclosed, for example, in Japanese Patent Laid-Open Publication SHO 62-205124(1987) and 62-241923(1987).
Some polyimides are thermoplastic and articles such as fibers, films, sheets, electric wires, bars, plates and pipes can be formed by common extrusion processes.
The common extrusion processes, however, have not yet been satisfactory for the production process of these articles from the above polyimides.
The polyimides consisting of recurring units represented by the above formula (I) have better extrudability as compared to other difficultly processable polyimides and are usually processed into pellets and extruded to form articles.
However, when the pellets thus obtained are fed as such to the extruder, extrusion cannot be smoothly operated, bubbles and die tailings are generated even though articles are obtained, and thus it is difficult to manufacture products having uniform quality.
The phenomenon results from the fact that the resin charged to the extruder is subjected to rapid plastization by heat and pressure at the feed zone and/or the compression zone of a screw. The resin sticks to the screw alone and idly slips on the internal surface of the cylinder after a certain number of revolutions of the screw. Thus, the resin cannot be extruded at all in some cases. Even in other cases where the resin is not blocked, a gas, for example, the gas transferred with the rapidly plasticized resin is not removed in the compression zone of the screw where the air shall be essentially excluded. The air thus entrained in the die is found to generate bubbles in the extrudate. For example, in fiber processing, the phenomenon tends to cause problems such as denier variation resulting from fluctuation of the extruded amount and end breakage due to bubble generation. In film and sheet processing, the phenomenon is liable to result in problems such as thickness fluctuation due to variations of the extruded amount and defective appearance or decrease in mechanical strength caused by generation of bubbles and die tailings. Further, in bar and pipe processing, the phenomenon is apt to cause problems such as fluctuation of diameter resulting from variations of the extruded amount, decreases in strength due to generation of internal bubbles and defective appearance, caused by die tailings. Additionally, in wire processing, the phenomenon is liable to cause problems such as reduction of electrical insulation properties due to the generation of bubbles and die tailings.
A countermeasure for these problems is a method for lowering the cylinder temperature of the extruder to 250 C. or less, that is, a temperature for inhibiting rapid plastization of the resin. The method, however, is unfavorable because the viscosity of the molten resin becomes abnormally high and renders extrusion impossible, or leads to deterioration of the resin resulting from a large amount of heat build-up by shear. Another countermeasure considered is modification of screw configuration. However, fluctuation of the extruded amount, that is, so-called surging takes place and renders extrusion impossible in some cases. Hence, it is difficult to solve the problems.
As stated above, the thermoplastic polyimides still have problems to be solved in the extrusion process. Unless these problems are overcome, it will be difficult to obtain articles having the required performance.